Phase sensitive acquisition and reconstruction, such as an “in-phase sand out-of-phase”-method of which the Dixon method may be best known, see e.g. W. T. Dixon, “Simple proton spectroscopic imaging,” Radiology, vol. 153(1), pp. 189-194, 1984, has many advantages. Dixon imaging provides two images, one showing the fat content in each voxel and one showing the water content in each voxel. Hence Dixon images are not adversely affected by partial volume effects. Separate water and fat images are also useful in the segmentation process. In short, Dixon imaging is performed by acquiring two separate images: one where the signals from fat F and water W are out of phase (OP=W−F) and one where they are in phase (IP=W+F). Ideally, water and fat thus can be obtained from the sum and difference of these images, respectively, and the total fat content in any region of interest can be calculated as the integral of the fat image over that region.
A chemical shift artifact manifested as a water fat shift artifact causes distortion in water and fat images obtained using phase sensitive acquisition and reconstruction, as in the Dixon method. The resonance frequency of protons in human methylene lipid [CH2]n and water differ by 3.5 ppm corresponding to 224 Hz at a field strength of 1.5 T. This intrinsic difference can be utilized for effective fat and water separation using the phase sensitive acquisition and reconstruction. However, a consequence from utilizing resonance frequency shift in such methods is spatial misregistration in the frequency encoding direction known as the chemical shift artifact, in the case of fat and water images, the water-fat shift artifact. In magnetic resonance imaging (MRI) the frequency is used to encode the spatial position of the signal. As the RF-pulse is tuned at the frequency of water, fat will have a relative frequency shift that cannot be distinguished from the phase difference introduced by the frequency encoding. The water-fat shift artifact typically appears close to fat structures and is caused by the bi polar gradient being used.
A flyback protocol has been proposed, see e.g. Cunningham. Magnetic Resonance in Medicine, 2005. p. 1286-1289, to eliminate the problem as the misregistration is constant between acquisition times, but has the effect of decreasing the signal to noise ratio (SNR).
The effect of the water fat shift artifact is typically not negligible in phase sensitive reconstruction such as Dixon imaging. Artifacts originating from the water fat shift should thus be handled in order to take full advantage of the highly effective fat and water suppression and high SNR that is enabled by the technique. A specific example of a situation where the artifact become important is during the examination of thin structures or organ surfaces surrounded by fat where the clinical question often is to judge if pathologic tissue penetrates the structure or not.
Intensity inhomogeneity prevalence in MRI is due to factors such as static field inhomogeneity, RF excitation field non-uniformity, in homogeneity in reception coil sensitivity and patient movement. The effect of the non-uniformity is usually a slow varying non anatomic intensity variation over the image. Although it sometimes can be difficult to see the intensity non-uniformity by visual inspection there are implications that significantly can decrease segmentation and registration results as many medical imaging techniques is based on the assumptions that the same tissue has the same intensity throughout a volume. More importantly it affects the linear quantification of the MR signal. A voxel containing a certain amount of fat should have the same signal strength, independent of where it is located in space. This is not true in case of intensity inhomogeneity occurrence.